Starting of Induction Motors

STARTING OF INDUCTION MOTORS

When the 3‒phase supply is connected to the stator of a three‒phase induction motor, a rotating magnetic field is produced and the rotor starts rotating. Thus, a three‒phase induction motor is self‒starting. At the time of starting the motor slip is unity and the starting torque is very large. The purpose of a starter is not only to start the motor but it also performs the following functions:

(i) Reduction of the heavy starting current

(ii) Overload and no‒load protection

In general, 3‒phase induction motors may be started either by connecting the mote directly to the full voltage of the supply or by applying a reduced voltage to the motor starting period. The torque of an induction motor is proportional to the square of the applied voltage. Thus, a greater torque is exerted by a motor when it is started on full voltage than when it is started on reduced voltage.

 Some of the starters for starting induction motors are discussed below:

Squirrel Cage Induction Motors

The following are the commonly used starters for cage motors.

1. Direct‒online starter

2. Star‒Delta starter

3. Auto transformer starter

1. Direct‒online Starter

In the direct on line method of starting cage motors, the motor is directly connected to the 3‒phase supply. Fig. 5.22, shows the connection for direct‒on‒line [D.O.L] Starter.

It consists of a coil‒operated contactor C controlled by start and stop push button switch which may be installed at convenient places remote from the starter. On pressing the START push button S₁, the contactor coil C is energised from two line conductors L₁ and L₂. The three main contacts M and the auxiliary contact A close and the terminals a and b are short‒circuited.

The motor is thus connected to the supply. When the pressure on S₁ is released, it moves back under spring action. Even then the coil C remains energised through ab. Thus, the main contacts M remain closed and the motor continues to get supply. For this reason, contact A is called hold‒on‒contact.

When the stop button S₂ is pressed, the supply through the contactor coil C is disconnected. Since the coil C is de‒energised, the main contacts M and auxiliary contact A are opened. The supply to the motor is disconnected and the motor stops.

2. Star‒Delta Starter

A Star‒Delta starter is used for cage motors designed to run normally on delta connected stator winding. Fig [5.23 (a)] shows the connections of a 3‒phase induction motor with a Star‒Delta starter. When the switch 's' is in the START position, the stator windings are connected in STAR Fig. [5.23 (b)]. When the motor picks up speed, say 80% of its rated value, the change over switch is thrown quickly to the RUN position which connects the stator windings in DELTA [Ref Fig. 5.23 (c)].

By connecting the stator windings, first in star and then in delta, the line current drawn by the motor at starting is reduced to 1/3^rd as compared to starting current with the windings connected in delta. At the time of starting when the stator windings are star connected, each stator phase sets a voltage V_L/√3 where V_L is is the line voltage. Since the torque developed by an induction motor is proportional to the square of the applied voltage, star‒delta starting reduces the starting torque to 1/3^rd of that obtained by direct‒delta starting.

Relation between T_st and T_f

 I_st per phase 1/√3I_sc per phase

where, I_sc is the current per phase which delta‒connected motor would have taken if switched on to the supply directly.

However, the line current at start is equal to 1/√3 of line current, I_sc

Starting torque,

 T_st ∝ I_st² at s=1                  ………….. (5.29)

Full‒load torque,

T_f ∝ I_f²/s_f               ………….. (5.30)

Eqn (5.29) divided by eqn (5.30), will give,

This type of starters are employed for starting 3‒phase squirrel cage motors of rating between 4 and 20 kW.

3. Auto Transformer Starter

An auto transformer starter is suitable for both Star and Delta connected motors. In this method, the starting current is limited by using a 3‒phase auto transformer to reduce initial stator applied voltage. Fig. 5.24 shows the motor with the auto‒transformer starter. The auto transformer is provided with a number of tapping arrangements.

Here, a double throw switch is used to connect the auto‒transformer in the circuit for starting. When the handle H of the switch is in the START position, the primary of the auto transformer is connected to the supply line and the motor is connected to the secondary of the auto‒transformer.

When the motor picks up speed, say to about 80% of its rated value, the handle H is quickly moved to the RUN position, the auto‒transformer is disconnected from the circuit and the motor is directly connected to the line and gets its full rated voltage. The handle is held in the RUN position by the under‒voltage relay. In case the supply voltage fails or falls below a certain value, the handle is released and returns to the OFF position.

Slip Ring Motor

1. Rotor rheostat starter

1. Rotor - Resistance Starter [Slip‒ring Motor Starter]

In this method, a variable star‒connected rheostat is connected in the rotor circuit through slip‒rings and full voltage is applied to the stator winding as shown in Fig. 5.2

At starting period, the handle of rheostat is set in the OFF position so that maximum resistance is placed in each phase of the rotor circuit. This reduces the starting current and at the same time starting torque is increased.

As the motor picks up speed, the handle of rheostat is gradually moved in clockwise direction which cuts out the external resistance in each phase of the rotor circuit. When the motor attains normal speed, the change‒over switch is in the ON position and the whole external resistance is cut out from the rotor circuit.